1. Field of the Invention
The present invention relates to a buffer amplifier for amplifying an oscillation signal of an oscillator used in a portable telephone or the like.
2. Related Art
A communication frequency band used by a portable telephone differs according to communication systems. For example, it is 900 MHz in the frequency division multiple access (FDMA) and 2.29 GHz in the code division multiple access (CDMA).
A buffer amplifier in a conventional portable telephone which is used for, for example, both the FDMA and CDMA will be described with reference to FIG. 4.
In FIG. 4, the buffer amplifier comprises a transistor 31 for amplifying the level of an inputted oscillation signal, an inductor 32 which has an end connected to the collector of the transistor 31 so as to supply a voltage to the collector of the transistor 31, and capacitors 33 and 34 which are connected in parallel to the inductor 32 in a high frequency receiving manner.
A source voltage input terminal 35 is connected to the other end of the inductor 32 so that a voltage is supplied to the collector of the transistor 31 via the inductor 32. The capacitors 33 and 34 which are connected in series at their one ends are coupled to the collector of the transistor 31. The other end of the capacitor 33 is connected to the collector of the transistor 31 and the other end of the capacitor 34 is connected to the emitter of the transistor 31.
The connection point of the capacitors 33 and 34 is grounded via a change-over switch 36.
An oscillator change-over switch 38 is connected to the base of the transistor 31 via a coupling capacitor 37. Two oscillators 39 and 40 having different oscillation frequencies are connected to the oscillator change-over switch 38. The oscillator 39 produces oscillation at the frequency (900 MHz) of the FDMA and the oscillator 40 produces oscillation at the frequency (2.29 GHz) of the CDMA.
A by-pass capacitor 41 and a resistor 42 which are connected in parallel are coupled to the emitter of the transistor 31 and one ends of the by-pass capacitor 41 and the resistor 42 are connected to the ground.
Resistors 44 and 45 which are connected in series at their one ends are coupled to the connection point of the inductor 32 and the source voltage input terminal 35. The other end of the resistor 45 is grounded. A bias voltage is applied to the base of the transistor 31 from the connection point of the resistors 44 and 45.
A mixer 47 is connected via a capacitor 46 to the connection point of the collector of the transistor 31 and the capacitor 33.
For example, in case of using the circuit in the FDMA, the oscillator change-over switch 38 is changed over so that an oscillation signal is supplied from the oscillator 39 to the base of the transistor 31 and the change-over switch 36 is turned on.
Consequently, a parallel resonance circuit is constructed by the inductor 32 and the capacitors 33 and 34 which are connected in parallel in a high frequency receiving manner. By tuning the resonance frequency of the parallel resonance circuit to the fundamental harmonic (900 MHz) of the oscillation signal of the FDMA, only the fundamental harmonic in the oscillation signal of the FDMA is outputted to the mixer 47.
In case of using the circuit in the CDMA, the oscillator change-over switch 38 is changed over so that an oscillation signal is supplied from the oscillator 40 to the base of the transistor 31 and the change-over switch 36 is turned off.
Consequently, a parallel resonance circuit is formed by the inductor 32 and the capacitor 33. By tuning the resonance frequency of the parallel resonance circuit to the fundamental harmonic (2.29 GHz) of the oscillation signal of the CDMA, only the fundamental harmonic of the oscillation signal in the CDMA is outputted to the mixer 47.
In the conventional buffer amplifier as described above, since only fundamental harmonics of the two oscillation signals required in both of the FDMA and CDMA are amplified, two oscillators are necessary and the cost is accordingly high.